Metal finishing effluents, generated in the application of cleaning, oxide and scale removal, electroplating, etching, metal stripping, etc., are problem materials in that they are toxic to aquatic life, to the biota employed in sewage treatment plants, contaminate ground water, etc. For these reasons, regulatory requirements strictly limit the residual metal content of treated effluents. In general, the waste treatment efforts directed toward this end are based upon chemical precipitation inasmuch as most metals have only a limited solubility at elevated pH when converted to the hydroxides, carbonates or oxides of the particular metal of concern. It is well known that because of an unusually variable and complex ionic background, the long time needed to approach equilibrium conditions, the presence of organic complexing agents in most of the waste streams, etc., a number of waste treatment plants cannot achieve the low levels of metal residuals stipulated by the regulatory requirements. Research work is and has long been conducted to solve this problem all over the world and technical literature is replete with numerous recommendations for its solution.
In general, four approaches for the reduction of the residual metal content of metal finishing effluents can be distinguished.
In one, removal by adsorption, various silica and alumina clays in both natural and purified forms have been employed as a means for the final treatment of metal-containing effluents. It has also been found that some organics such as cellulosic materials, peat moss, bacterial colonies in sewage treatment plants, etc., are capable of similar adsorption effects.
In a second approach, ion exchange resins have been used exchanging ions such as hydrogen or sodium for heavy metal ions that are held more tightly within the resin's molecular structure. Some of the natural clay minerals have the required molecular configuration and, under the name of natural zeolites or green sand, have been the forerunners of the polymerized organic resins commonly used today.
A third approach is based on the observation that many of the metal finishing process solutions contain metals in an organic molecular complex, e.g., as a chelate. Such non-ionic soluble metal complexes do not adsorb on the media usually employed. Peat moss has been sulfonated and, in another development, starch xanthate to release into the solution a sufficient volume of soluble sulfide compounds to react with the metal complex and allow the generation of a metal-sulfide that is adsorbable on the peat moss or starch or xanthated cellulose.
A fourth approach is based on the known fact that the solubility of the metal-sulfide compounds is significantly lower than the residual solubility of the same metal as a hydroxide, carbonate, or their combination with hydrated oxides gained from a conventional or high pH "neutralization" reaction. Precipitation with sulfide chemicals has therefore been studied as a means to reduce the soluble residuals of the ionized metal compounds and to render insoluble some of the metals held in the organic chelate complexes. A recent report from such a study supported by the U.S. Environmental Protection Agency reports the results of various commonly practiced neutralization approaches when combined with the addition of sulfide chemicals; "Sulfide Precipitation of Heavy Metals"; A. K. Robinson and J. C. Sum; U.S. EPA-NITS EPA-600/2-80-139. Hence, precipitation with sulfide chemicals has been employed to reduce the soluble residuals of the ionized metal compounds and to render insoluble some of the metals held in the organic chelate complexes.
The "Sulfex Process", U.S. Pat. No. 3,740,331, is based on the addition of a slightly soluble metal-sulfide compound, to limit the soluble sulfide concentration in a waste stream. This is accomplished by either adding the slightly soluble metal sulfide or by adding the soluble alkali-metal sulfide and the metal salt separately and reacting the added metal salt with the sulfide simultaneously with the primarily desired reaction with the metals dissolved in the effluent. The metal preferentially employed as the sulfide source is either a ferrous or manganese salt and it is to be in excess of the total reacting sulfide present to overwhelm the toxic metals present and remove them by coprecipitation.
However, experience has shown that none of the suggested methods adequately serve the purposes of industry.
With respect to the first and second approaches, it has been found that the adsorption ion exchange media investigated could not remove the metals under investigation from even the more labile inorganic complexes such as ammonium.
With respect to the third approach, sulfonated peat moss, xanthated starch or cellulose, etc., performed erratically when attempting metal removal from some of the tight metal chelates such as EDTA. Another serious shortcoming of such processes is that with the use of a bulky adsorption medium, the volume of the generated sludge has been increased manyfold.
With respect to the fourth approach, it has been recognized that the nature of the metal complex makes a very significant difference. While good results are obtained for certain ionized metal salts or metals in a more labile inorganic or organic complex, such as ammonia, acetate, gluconate, or NTA, the metals in an EDTA chelate are hardly affected. The cited EPA study did not recognize this distinction and provided no explanation for the apparent variability of their test results. Another problem with this approach is that the precipitated metal sulfides form a very fine precipitate, most often colloidal in nature, requiring the subsequent addition of a coagulant to clarify the suspended solids content or filtration. Furthermore, when sulfide additions are made to a waste stream, the sulfide has to be added in excess to the stoichiometric requirements, to provide the necessary driving force for as complete a precipitation reaction as can be achieved. This can easily leave an excessive concentration of free sulfide in an effluent stream which would be objectionable for a direct discharge.
It has therefore been our research effort to find the optimum physical and chemical conditions for sulfide precipitation and to eliminate the unpredictable variations in the chemical reaction and to develop a process suitable for the separation of metals from the solution, even when held in a tight chelate such as that with EDTA.